Seat occupancy by the passengers in motor vehicles is an important input variable for a plurality of technical applications in motor vehicles. This applies especially to occupant restraint systems for which efficient use often depends on the sitting position of the motor vehicle occupants. Sensor seat mats consisting of a plurality of pressure-sensitive sensor elements are used for seat occupancy detection in motor vehicles. This are distributed over the surface of the motor vehicle seat into columns and rows and can thereby record the effects of pressure exerted by a person or an object on the seat surface. The individual pressure-sensitive sensor elements of a sensor seat mat mostly consist of pressure-dependent electrical resistors. The pressure acting on the seat surface of the motor vehicle seat is determined by a downstream electrical control unit. This operates by recording the change in electrical resistance of the sensor elements distributed over the seat which provides feedback about the person occupying the seat.
Such sensor arrangements embodied as sensor mats are known from DE 200 14 200 U1 and from the article, Occupant Classification System for Smart Restraint System’, Society of Automotive Engineers Inc. of 1999, BNSDOCID XP002184965. When these types of occupant detection systems are used the likelihood of a malfunction must be kept as low as possible. In the event of a malfunction of the occupant detection system there is the danger of serious injury to the vehicle occupants to be protected, since the occupant detection system mostly has a direct influence on the trigger behavior of the occupant restraining means in a motor vehicle.
For example, an undetected increase in line resistance in the current path to the sensor elements of the sensor seat mat could falsify the determination of the resistance of the sensor elements, which would incorrectly determine an occupant weight signal and would be used as a basis for the further evaluation of the occupant detection system. The weight of the vehicle occupants incorrectly determined in this way or even their incorrect classification for example as a particularly large and heavy person could lead to the control unit of the vehicle protection systems releasing an airbag intended to protect the occupants when this is more likely to injure the actual occupants on the vehicle seat.
This is why checks are already usually made at regular intervals on the line resistances of the electrical supply lines to the sensor elements in sensor arrangements for seat occupancy detection. To this end resistor components are arranged in the supply lines to the rows and columns of sensor elements of a sensor seat mat which, together with the electrical lines, form the overall resistance of a supply lead of sensor elements of a sensor seat mat. The basic resistance of each supply lead to sensor elements of a sensor seat mat with the resistor components included within it is determined by measurement and compared with a reference value.
This reference value is usually stored in a memory medium of the occupant detection system and where necessary is made available to a control unit. For example, the control unit requests the reference value stored in an EEPROM of the occupant protection system in order to subsequently compare it with the measured value.
The reference value serves as a limit value of a measurement of the overall resistance of the supply lead. If the overall resistance exceeds the reference value this is detected for example by a downstream control unit of the sensor seat mat.
This procedure is executed on a regular basis, for example before each measurement of the sensor elements, but at least in the runup phase of the occupant detection system after vehicle startup.
The violation of the reference value (of the limit value) indicates possible damage and thus the danger of a possible malfunction of the sensor seat mat, which can possibly result in an incorrect trigger function of the occupant restraining means.
If the occupant protection system detects an error in the sensor seat mat in this way, this information can then be used for example to deactivate the occupant protection system or to limit its use. An occupant detection system error is then usually indicated to the vehicle occupants with the aid of an airbag warning light on a display panel in the vehicle.
The overall resistance of the supply lead to the sensor elements of a sensor arrangement and especially to the rows and columns of sensor elements of a sensor seat mat is also referred to below as the overall resistance of a row or also as the overall resistance of a column.
Suitable resistance components for this leakage or interruption detection measurement at a sensor seat mat are preferably arranged integrated on the sensor seat mat and are similar in structure to the structure of pressure-sensitive sensor elements.
The measuring accuracy however makes monitoring the overall resistance of the supply lead to the sensor elements over the entire operating life of a motor vehicle an extremely difficult measuring task. For the most part the accuracy obtained in determining the line resistance to the sensor elements is too small for a meaningful monitoring of the overall resistance of the supply lead to the sensor elements of the sensor seat mat.
On the one hand this measurement accuracy is conditional on an inaccurate evaluating electronics for recording the overall resistance as a measurement variable.
To keep the costs of the occupant detection system as low as possible, it is however preferable to look for technical solutions in which these inaccurate but nonetheless cost-effective electronics can be employed.
On the other hand the resistance components used vary very widely in their resistance values as a result of manufacturing tolerances.
Account can be taken of the manufacturing-related range of variations in the resistance values of the resistance components by determining the overall resistance value of each individual resistance component together with its line resistances when the sensor seat mat is manufactured and subsequently storing these values in a memory. For a later comparison measurement the stored values for the resistance component are then compared with the current resistance value measured.
However this does not adequately take account of a systematic change of the resistance value of the resistance component during the course of its lifetime within the sensor arrangement in a vehicle seat. The resistance value of the resistance component can change completely during the course of its lifetime. If this causes the overall resistance of the resistance component with its lines to exceed a previously stored reference value, the control unit of the sensor seat mat would if necessary also detect an error in the supply lead to the sensor elements if the change had been effected exclusively by this systematic change of the overall resistance value. This might possibly cause a sensor seat mat to be interpreted as faulty and cause an airbag warning lamp to come on for no reason.
The costs of an unnecessary repair or replacement of a fault-free sensor seat mat are not only unnecessary but are also mostly not insignificant and should therefore be avoided if possible.
One improvement to the previous methods of monitoring of the resistance of the electrical supply lead of sensor elements is represented by monitoring the deviations from the stored reference values, whereby the deviation of the measured overall resistance for each resistance component is compared with the deviation for all other resistance components. If the deviation of the overall resistance for a resistance component differs significantly from that of the others it is not a case of a systematic change of the overall resistance but indicates a fault within the electrical supply lead involved. With this method of operation all systematic changes of the resistance components and their leads are taken into account which might for example be caused by temperature variations and material ageing etc.
Advantageously this also accordingly takes account simultaneously of measurement electronics faults both in the initial determination of the reference values and also for the comparison measurements made subsequently. The danger of incorrect weight interpretation by the occupant protection system is reduced by a significant amount with this method.
The two methods described are however extremely expensive to manufacture and generally not possible to implement on grounds of cost.